1. Field of the Invention
The present invention relates to methods of operating and controlling systems for air conditioning systems and, more particularly, to a method of operating and controlling a system for balancing the load of a plurality of chiller units in a chiller plant to improve the efficiency and reliability of the chillers.
2. Description of Related Art
Generally, large commercial air conditioning systems include a chiller which consists of an evaporator, a compressor, and a condenser. Usually, a heat transfer fluid is circulated through tubing in the evaporator thereby forming a heat transfer coil in the evaporator to transfer heat from the heat transfer fluid flowing through the tubing to refrigerant in the evaporator. The heat transfer fluid chilled in the tubing in the evaporator is normally water or glycol, which is circulated to a remote location to satisfy a refrigeration load. The refrigerant in the evaporator evaporates as it absorbs heat from the heat transfer fluid flowing through the tubing in the evaporator, and the compressor operates to extract this refrigerant vapor from the evaporator, to compress this refrigerant vapor, and to discharge the compressed vapor to the condenser. In the condenser, the refrigerant vapor is condensed and delivered back to the evaporator where the refrigeration cycle begins again.
To maximize the operating efficiency of a chiller plant, it is desirable to match the amount of work done by the compressor to the work needed to satisfy the refrigeration load placed on the air conditioning system. Commonly, this is done by capacity control means which adjust the amount of refrigerant vapor flowing through the compressor. The capacity control means may be a device for adjusting refrigerant flow in response to the temperature of the chilled heat transfer fluid leaving the coil in the evaporator. When the evaporator chilled heat transfer fluid temperature falls, indicating a reduction in refrigeration load on the refrigeration system, a throttling device, e.g. guide vanes, closes, thus decreasing the amount of refrigerant vapor flowing through the compressor drive motor. This decreases the amount of work that must be done by the compressor thereby decreasing the amount of power draw (KW) on the compressor. At the same time, this has the effect of increasing the temperature of the chilled heat transfer fluid leaving the evaporator. In contrast, when the temperature of the leaving chilled heat transfer fluid rises, indicating an increase in load on the refrigeration system, the throttling device opens. This increases the amount of vapor flowing through the compressor and the compressor does more work thereby decreasing the temperature of the chilled heat transfer fluid leaving the evaporator and allowing the refrigeration system to respond to the increased refrigeration load. In this manner, the compressor operates to maintain the temperature of the chilled heat transfer fluid leaving the evaporator at, or within a certain range of, a setpoint temperature.
Large commercial air conditioning systems, however, typically comprise a plurality of chillers, with one designated as the "Lead" chiller (i.e. the chiller that is started first and stops last) and the other chillers designated as "Lag" chillers. The designation of the chillers changes periodically depending on such things as run time, starts, etc. The total chiller plant is sized to supply maximum design load. For less than design loads, the choice of the proper combination of chillers to meet the load condition has a significant impact on total plant efficiency and reliability of the individual chillers. In order to maximize plant efficiency and reliability it is necessary to optimize the selection and run time of the chillers' compressors, and insure that all running compressors have equal loading. The relative electrical energy input to the compressor motors (% KW) necessary to produce a desired amount of cooling is one means of determining the balance of a plurality of running compressors. However, if the building load changes and the temperature of the chilled water supplied to the building from the chiller plant deviates from the desired chilled water setpoint, then the Lead chiller changes capacity, thus power draw also changes, to return the chilled water temperature to the set point. However, the lag compressors, in an attempt to maintain balance, also change capacity and overcompensate for the change in load, which in turn causes the Lead compressor to change capacity again. Accordingly, the desired balance among chillers in normally not attained. Thus, in the prior art chiller load balancing was normally left to chance. Each individual lag chiller would attempt to control its own discharge water temperature to a setpoint which was presumed to be the same as the lead chiller, but in fact could be subject to substantial variation and cause the relative % KW, or loading factor, of the operating chillers to vary correspondingly. Chillers usually operate most efficiently when they are near full load conditions. Having some chillers fully loaded while others are partially loaded, i.e. unbalanced, leads to inefficient system operation. Thus, there exists a need for a method and apparatus which balances the chiller loads and which minimizes the disadvantages of the prior control methods.